In the event of a military strike against this country, it can be expected that our offensive and defensive missile systems would be prime targets for enemy attack. Thus, the sheltering of these systems from the effects of such an attack is accordingly of great concern.
Among other alternatives, it is contemplated that our missile systems should be built inside large underground caverns at extreme depths below the surface, beyond the destructive effects of an enemy warhead. For example, it is anticipated that in a major strike on a missile installation, the first one thousand feet or so of surface soil and rock would be turned into rubble. Accordingly, it is suggested that the caverns be constructed at depths on the order of 3,000 feet below the surface. After the attack, tunnels could be burrowed upwardly through the earth and rubble, effecting communication with the surface and enabling the launch of a possible counterattack by our missiles.
Numerous kinds of drilling systems have been developed to accomplish a variety of drilling and tunneling objectives. For example, conventional oil well drilling rigs utilize a rock crushing bit mounted on the end of a drilling string made up of sections of pipe. A rotating "table" mounted on the rig platform slidably engages the drill string, and provides the rotational motion necessary for the bit to fracture the rock that it encounters. Drilling fluid is pumped through the drill string and out of the bit, where the fluid acts to clean the bit and carry the crushed rock back up to the surface through the hole that has been drilled. The sliding engagement of the rotating table with the drill string enables the entire string to extend downwardly under the influence of gravity as the bit progressively "makes hole". Additional sections of pipe can be added to the string as needed, to achieve the desired hole depth.
An alternative drilling method utilizes a fluid driven drill bit, e.g. a positive displacement motor. In this arrangement, the circulation of the fluid pumped down the drill string causes a specially designed bit to rotate, and thus no rotating table is needed to turn the entire drill string.
Both of these methods provide bit rotation, but rely on gravitational force to achieve their downward drill string travel. While both methods have been successfully used in slant drilling applications, and have achieved downwardly drilled holes at angles of 45.degree. from the vertical and greater, they are not structurally suited for vertically upward drilling. Thus, neither method would be usable in the contemplated system requiring vertical penetration.
Machines for boring or tunneling upwardly are well known in the art, and are often referred to as raise-boring or box hole machines. Representative of many of the mechanisms used is the system disclosed in Webb U.S. Pat. No. 4,114,698, which provides an improved system for connecting new sections of pipe to a drill stem, while maintaining the vertical position of the drill string and cutting head. Another prior art patent, Still U.S. Pat. No. 4,179,001, shows a mechanism for installing a liner or casing in a vertical shaft concurrently with the drilling of the shaft. Jinno, et al., U.S. Pat. No. 4,248,312, discloses a full-face boring system with simultaneous installation of a side wall shield.
While these prior art systems to indeed provide a means for the drilling of a vertical hole, they all require a piecemeal assembly of the segmented drill stem to extend their length in an upward direction. Such assembly is laborious and time-consuming, and requires large areas within the underground chamber for the storage of the equipment and drill pipe needed. Drilling time is further increased by the fact that drill head servicing requires complete, piecemeal breakdown of the drill string for retrieval of the bit, followed by reassembly of the string in the original manner.